Photocatalytic Composites Containing Titanium and Limestone

ABSTRACT

New photocatalytic product comprising compounds of titanium integrated with limestone. The product is obtained by reacting limestone with a suitable precursor of titanium dioxide in a basic solution, followed by accurately washing the solid obtained, drying it and calcining it. A composite is obtained containing limestone, titanium dioxide and calcium titanate. The composite thus obtained, used as such or in mixture with other components, has shown an unexpectedly high photocatalytic activity.

FIELD OF THE INVENTION

The present invention concerns the field of photocatalytic materialsused for decontamination from environmental pollutants, and forpreserving the original colour of articles of manufacture exposed tosaid pollutants, with application in particular in the field of cement.

STATE OF THE ART

The use of titanium dioxide in the form of anatase as photocatalyst incement compositions is widely known. The resulting compositions areexploited to make various construction elements and articles ofmanufacture endowed with photocatalytic properties, capable ofdecomposing environmental pollutants in presence of light and oxygen. Inthese compositions titanium dioxide can be dispersed in bulk with theremaining components (WO-A-9805601, to the Applicant); alternatively,firstly a cement base free from titanium dioxide is formed, and then itis externally coated with titanium dioxide, optionally mixed withbinders and/or adhesives of various types. In all these cases thetitanium-containing photocatalyst is present in the form of a merephysical mixture with the mineral components of the cement composition.The interaction that is established in these cases is of the mechanicalor weakly electrostatic kind, and thus there is no adequate continuitybetween photocatalyst and rest of the mixture. This can lead to variousproblems linked to inadequate interpenetration of the photocatalyticcomponents and of those constituting the inert material. The closeinteraction between photocatalyst and mineral elements of the cement ishowever important for an effective photocatalytic action: indeed, inphotocatalytic cements the cement component is known to simultaneouslyabsorb atmospheric pollutants through a process of rapid dynamicequilibrium with the environment (adsorption/desorption): the pollutanttemporarily adsorbed is then decomposed by the photocatalyst. However,in known products the adsorbent part and the photocatalytic part areclearly distinct: in this situation a part of the adsorbed pollutant canbe desorbed before the photocatalyst is able to act sufficiently, withthe consequence of an insufficient level of photocatalysis.

In an attempt to improve the degree of interaction betweenphotocatalytic part and inert part, some materials have been proposed inwhich the titanium dioxide is supported on mineral components. Anexample of these products is titanium dioxide supported on metakaolin,described in patent application MI2007A002387, to the Applicant.However, as also highlighted in the aforementioned application inreference to various supports, the reactivity of the titanium dioxidevaries greatly from depending on the support, and the properties of theresulting product are extremely variable and often unsatisfactory.

A high-performance photocatalytic is particularly desirable in the caseof cement materials, characterised by a very low cost/weight ratio: forthese materials, any increase in production costs linked to the additionof fine additives reflects greatly on said ratio, risking to make theend product unmarketable.

Currently the need for photocatalytic composites in which thephotocatalytic part is greatly integrated with a mineral supportmaterial, which are endowed with a high photocatalytic activity, andpossibly obtainable through a low-cost production process, thus remainswidely unfulfilled.

Calcium titanate is a material with properties of refractoriness,chemical resistance and of a semi-conductor. It is found in nature invarious forms (e.g. perovskite) characterised by a mixture of phaseswith different ratios between calcium and titanium, e.g. CaTiO₃,Ca₃Ti₂O₇, Ca₄Ti₃O₁₀, CaTi₄O₉, CaTi₂O₅, Ca₂TiO₄, CaTi₂O₄(OH)₂, etc. Itcan be prepared via dry- or wet route. Dry preparation is generallycarried out by reacting titanium oxide and calcium carbonate attemperatures greater than 1300° C. (Izv. Akad. Nauk USSR-Neorg. Mater.,11 (1975) 1622). Wet preparation can be carried out in different ways,e.g. hydrothermically by heating an aqueous suspension of titanyloxalate and a hydrated titanium gel to 150-200° C. in an autoclave (T.R. N. Kutty and R. Vivekanandam, Mater. Lett., 5 (1987) 79-83). It isalso known to obtain calcium titanate via peroxide route by treating anaqueous calcium chloride and titanium chloride solution with hydrogenperoxide and ammonia, and subsequently calcining the precipitateobtained. (Pfaff, J. Eur. Ceram. Soc., 9, 1992, 293-299).

Mixtures of cement and titanates have occasionally been described. Forexample JP2000226248 describes cement mixtures with good flame and acidresistance containing a ceramic powder that includes potassium titanateand titanium dioxide.

SUMMARY

A new photocatalytic composite has now been identified in which thetitanium is tightly and stably integrated with a mineral currently usedin the field of cement, which is limestone. The composite is obtained byreacting a precursor of titanium dioxide with limestone in basicsolution, followed by accurate washing, drying and calcining of thesolid obtained. The composite contains limestone, titanium dioxide andcalcium titanate, the latter characterised by two crystalline phases notknown up to now (herein characterised and referred to as CT2 and CT5).The composite thus obtained, which can be used as such or in mixturewith other components, has shown an unexpectedly high photocatalyticactivity.

DESCRIPTION OF THE FIGURES

FIG. 1: Trend of the specific BET surface according to the treatmenttemperature of the photocatalytic composite STCA06.

FIG. 2: Diffractogram of the composite STCA 06

FIG. 3: Diffractogram of the acid residue of the composite STCA06

FIG. 4: Images in bright field TEM of a calcite crystal and of themicro-nanocrystalline aggregates

FIG. 5: High-resolution image of a grain of CT2 phase.

FIG. 6: Fourier transform of the image of FIG. 5 corresponding to theplane [100] with the main periodicities shown.

FIG. 7: High-resolution image of the grain rotated by 34.7° around theperiodicity 0.99 nm (scale 2 nm)

FIG. 8: Fourier transform of the image of FIG. 7 corresponding to theplane [100 ] with the main periodicities shown.

FIG. 9: Images in bright field TEM of the crystals of phases CT2 (e1)and CT5 (a1, b1, c1, d1) (scale 50 nm).

FIG. 10: High-resolution image of a grain of CT5 phase obtained (scale 2nm)

FIG. 11: Fourier transform of the image of FIG. 10 with the mainperiodicities shown.

FIG. 12: High-resolution image of a grain of CT5 phase (scale 2 nm)

FIG. 13: Fourier transform of the image of FIG. 12 with the mainperiodicities shown

FIG. 14: High-resolution image of a grain of CT5 phase (scale 2 nm).

FIG. 15: Fourier transform of the image of FIG. 14 with the mainperiodicities shown

FIG. 16: Abatement of NO over CEN mortar according to the type ofphotocatalyst. As such: CEN mortar with only Italbianco cement. As suchCA-01: CEN mortar with Italbianco cement and limestone.

FIG. 17: Abatement of NO_(x) over CEN mortar for the composite STCA06with respect to the cement.

FIG. 18: Abatement of NO over CEN mortar according to the treatmenttemperature.

FIG. 19: Abatement of the ethylbenzene by the tested products inproportion to the amount of titanium present.

DETAILED DESCRIPTION

The photocatalytic composite object of the invention compriseslimestone, titanium dioxide and calcium titanate; the latter is presentin part in the known form of perovskite (traces) and in part in the formof two new crystalline phases, herein identified and characterised forthe first time, referred to as CT2 and CT5.

For the purposes of the present invention by calcium titanate incrystalline phase CT2 it is meant a crystalline chemical compoundcontaining calcium and titanium, present in molar ratio 1:2, havingempirical formula CaTi₂O₅, identified by the characteristic diffractionpeaks: (002) d(interplanar distance)=4,959; (210-202) d=2,890; (013)d=2,762 and (310-122) d=2,138. These peaks are indexed with anorthorhombic cell having the following reticular parameters: a=7.1 Å,b=5.0 Å, c=9.9 Å.

For the purposes of the present invention by calcium titanate incrystalline phase CT5 it is meant a crystalline chemical compoundcontaining calcium and titanium, present in molar ratio 1:5, havingempirical formula CaTi5O11, identified by the characteristic diffractionpeaks: (002) d=8,845; (023) d=4,217; (110) d=3,611 and (006) d=2,948.These peaks are indexed with an orthorhombic cell having the followingreticular parameters: a=3.8 Å, b=12.1 Å, c=17.7 Å.

For the purposes of the present application the crystallographicparameters for phases CT2 and CT5 herein indicated and claimed are meantto be variable within a range of about ±0.5 Å for the parameters of thecell a,b,c, and within a range of about ±0.05 for the interplanardistances d; similarly, the calcium:titanium molar ratios indicatedabove are meant to be variable by about ±10%.

The microstructural characteristics of phases CT2 and CT5 are widelyillustrated in the experimental part.

In the composites object of the invention, the amounts of calciumtitanate in CT2 phase and in CT5 phase are widely variable: in general,the CT2 phase is present in a greater amount than CT5; for example theweight ratio CT2:CT5 is at least 60:40, or at least 80:20, or at least95:5. In an embodiment of the invention the calcium titanate is presentexclusively in CT2 phase or exclusively in CT5 phase.

The aforementioned calcium titanate in CT2 and/or CT5 phase representsper se a particular embodiment of the present invention.

In the present composite, the calcium titanate is accompanied by othercomponents, in particular limestone and titanium dioxide, the latter inmixed anatase and rutile form. The limestone used to form the compositeis the one commercially available, preferably in finely divided form,also commercially available (example source: cava di Tinella (Fasano,Brindisi))

The BET surface area of the composite generally ranges from 10 to 150m²/g, with preferred values between 15 and 50 m²/g, e.g. between 20 and30 m²/g.

The process for obtaining the composites described here constitutes afurther aspect of the invention. It generally comprises reactinglimestone and a precursor of titanium dioxide in a basic aqueoussolution. The reactants can be added into the reactor in indifferentorder; preferably the limestone is contacted first with the basicsolution and then with the precursor. The precursor used is preferablytitanyl sulphate. In the process conditions the precursor converts inpart into titanium dioxide and in part into calcium titanate describedabove. Preferably, an amount of precursor is used corresponding to atheoretical content (i.e. calculated considering a total conversion ofthe precursor into TiO₂) of about 40% by weight with respect to thelimestone. The basic solution is made such through use, for example, ofNaOH. The reaction goes on for a time of between 45 and 90 minutes, at atemperature ranging from 20 and 80° C. At the end of the reaction, theresulting solid product is recovered from the solution, carefully washedwith water until neutrality, dried and calcined. If sodium-containingreactants are used (e.g. NaOH), the sodium residue in the solid productsubjected to drying must be less than 0.05% by weight (expressed as Na₂Oover the dry product); such a condition is easily obtained for exampleby washing the recovered product until neutrality.

The calcining preferably takes place at a temperature ranging from 300to 800° C., e.g. between 450 and 700° C.; particularly effectivephotocatalytic composites have been obtained by calcining at about 650°C. The choice of the best calcining temperatures highlights aparticularly unexpected aspect of the present invention. Indeed, it isper se known (and even experimentally confirmed, see FIG. 1) that theincrease in calcining temperature results in a reduction of the specificsurface of the calcined product; on the other hand, photocatalysis is atypical surface phenomenon and such phenomena are predictablydisadvantaged in conditions of low contact surface; surprisinglyhowever, the present invention has shown the opposite tendency, byidentifying the most photocatalytically active composites in the lowerranges of specific surface, obtainable by calcining in the uppertemperature ranges.

A further object of the present invention is the photocatalyticcomposite obtained through the process described above.

From the point of view of the elemental composition (as detectable byX-ray fluorescence and atomic absorption), the composites according tothe invention can be further characterised as follows:

Calcium (expressed as CaO) 20-50% Titanium (expressed as TiO₂) 15-68%Sulphur (expressed as SO₃)  2-12% Sodium (expressed as Na₂O) <0.05%L.o.I. (*)  3-40% (*): loss on ignition

Or more preferably as follows:

Calcium (expressed as CaO) 26.9% Titanium (expressed as TiO₂) 51.7%Sulphur (expressed as SO₃) 7.54% Sodium (expressed as Na₂O) <0.01% L.o.I. (*) 13.4%

The elemental composition given in the tables refers to the composite asa whole: such a composite comprises, in addition to calcium titanate,limestone, titanium dioxide and possible residues of the reactants usedfor the for titanate-forming reaction.

However, as can be seen from the tables, a characteristic of thecomposite is that it is substantially free from sodium residues (i.e.having a sodium percentage, expressed as Na₂O, below 0.05% by weightover the dry product). Such a characteristic, obtainable by carrying outprolonged and repeated washes of the reaction precipitate, isresponsible for the formation of significant amounts of titanium dioxidein the composite. Otherwise, composites obtained similarly, but withoutelimination of the sodium residues, were substantially free fromtitanium dioxide: the latter family of composites has specificapplication advantages and is the object of a co-pending application tothe Applicant.

As highlighted by the electron microscopy observations contained in theexperimental part, the titanium dioxide and the calcium titanate are inthe form of crystalline grains of a size of about 10-150 microns,closely connected to limestone grains. There is thus clearly a strongaggregative link between the photocatalytic portion of the composite(titanium) and the mineral support component (limestone); within theseaggregates, the calcium titanate crystals in phase CT2 are generallyrounded, whereas those in phase CT5 generally have a characteristic rodshape.

The present invention represents a successful example of compositematerial in which different titanium-containing compounds are closelyand stably linked to a support material (limestone) able to be used inthe cement field. The close interconnection between the photocatalyticand non-photocatalytic parts of the composite obtains a substantialcontinuity between absorption sites of the pollutants and decompositionsites thereof, with the advantage of high photocatalytic efficiency.Such efficiency has been highlighted by abatement tests of N-oxides(NO_(x)) and VOC (aromatic hydrocarbons), using the composite of theinvention either as such, or incorporated in bulk in a cement matrix.

A further object of the invention is therefore the use of thephotocatalytic composite described earlier as photocatalytic product assuch, or in the preparation of cements and cement articles ofmanufacture endowed with photocatalytic activity. The article ofmanufacture can contain the composite dispersed in bulk, or layered onits outer surfaces, as a coating: in the latter case the photocatalyticcomposite is preferably mixed with suitable tackifiers, used to promotesuitable cohesion between article of manufacture and coating layer. Inany case, the composite is used in amounts such as to obtain aconcentration of composite in bulk preferably ranging from 1% to 10%,more preferably between 2.5% and 8.5%. The methods for the dispersion inbulk or for the outer coating are per se widely known in the field inquestion.

An aspect of the invention thus concerns photocatalytic composition, inparticular of cementitious type, comprising the composite describedabove. The further elements of the cement composition are those commonlyknown, in particular: various hydraulic binders, optional aggregates andadditives used in the cement field. The hydraulic binders and theaggregates (defined for example by standards UNI ENV 197.1 and UNI 8520)are products widely known in the field. The compositions according tothe invention can be provided in fluid state, or else mixed with water(to form mortars or concretes, depending upon the granule size of theaggregates used), or else they can be provided in the correspondingforms free from water (dry premixes). These compositions are used toform photocatalytic articles of manufacture through suitable casting inmoulds and similar technologies; the resulting articles of manufacturecontain the composite of the invention dispersed in bulk. Alternatively,they can be used as coating formulations of pre-existing articles ofmanufacture, preferably co-formulated with suitable tackifiers.

The invention is illustrated hereafter not for limiting purposes throughthe following examples.

EXPERIMENTAL PART Example 1 Preparation of the Composite (STCA06)

45 g of a commercial calcareous filler (origin: cava Tinella diBrindisi) were stirred, suspended in 160 ml of a NaOH solution (200 g/lin distilled water), and an aqueous solution of 300 ml of TiOSO₄ (100g/l of TiO₂), so as to have a theoretical TiO₂ content equal to about40% by weight, was dripped. After centrifuging and washings withdistilled water the powder was dried at 105° C. in a ventilated oven.Before performing the calcining heat treatment at 450° C. for 2 hours,the product was broken up so as to obtain a powder. Further samples ofthe same filler were treated in the same way, calcining at 550 and 650°C.

Example 2 Microstructural Characterisation

The composite STCA 06 obtained in example 1 (calcining temperature 650°C.), subjected to diffractometric analysis (diffractometer BRUKER D8Advance and CuKα (λ_(Cu)=1.545 Å radiation) proved to be a polyphasemixture composed of calcite, traces of perovskite, titanium dioxide, andcalcium titanate in different crystalline phases. In particular, thediffraction profile showed the presence of a series of peaks notattributable to known crystalline phases, which can be referred to twodifferent phases that proved to be calcium titanate-containing“compounds” with Ca:Ti ratios of 1:2 and 1:5 respectively (FIG. 2).

The accurate position of the peaks of the new crystalline phases wasdetermined through diffractometric analysis of the sample afterelimination of the calcite by treatment in diluted HCl (1:10) andsubsequent drying at 60° C. (FIG. 3).

The observed interplanar distances (d) of the two phases are shown inthe following tables, wherein h,k,l indicate the Miller indices, and °2θindicates the diffraction angle.

interplanar distances for CaTi₂O₅; Space group: Pna2₁ a = 7.1 Å, b = 5.0Å, c = 9.9 Å h k l d °2θ 0 0 2 4.96 17.87 0 1 1 4.48 19.80 1 1 0 4.1021.66 1 1 1 3.79 23.46 2 0  0* 3.55 25.08 2 0 1 3.34 26.67 1 1 2 3.1628.22 2 1 0 2.90 30.84 2 0 2 2.89 30.97 2 1 1 2.78 32.16 0 1 3 2.7632.39 1 1 3 2.57 34.83 0 2 0 2.51 35.72 2 1 2 2.50 35.86 0 0 4 2.4836.19 2 0 3 2.42 37.14 1 2 0 2.37 37.98 1 2 1 2.30 39.09 0 2 2 2.2440.22 2 1 3 2.18 41.40 3 1 0 2.14 42.20 1 2 2 2.14 42.27 1 1 4 2.1242.57 3 1 1 2.09 43.22 2 2 0 2.05 44.15 2 0 4 2.03 44.54 2 2 1 2.0145.13 3 1 2 1.96 46.17 1 2 3 1.92 47.18 2 2  2* 1.89 47.99 2 1 4 1.8848.27 0 1 5 1.85 49.35 3 1 3 1.80 50.79 1 1 5 1.79 51.11 4 0 0 1.7751.48 0 2 4 1.76 51.77 4 0 1 1.75 52.36 2 2 3 1.74 52.48 2 0 5 1.7352.83 3 2 0 1.72 53.16 1 2 4 1.71 53.47 3 2 1 1.70 54.02 4 1 0 1.6754.85 4 0 2 1.67 54.94 0 0 6 1.65 55.54 0 3 1 1.65 55.63 4 1 1 1.6555.69 2 1 5 1.64 56.14 1 3 0 1.63 56.42 3 2 2 1.63 56.54 3 1 4 1.6256.78 1 3 1 1.61 57.25 4 1 2 1.58 58.17 2 2 4 1.58 58.36 4 0 3 1.5659.06 1 3 2 1.55 59.68 1 1 6 1.53 60.32 3 2 3 1.53 60.59 1 2 5 1.5260.87 2 3 0 1.51 61.16 2 0 6 1.50 61.87 2 3 1 1.50 61.95 0 3 3 1.4962.09 4 1 3 1.49 62.15 1 3 3 1.46 63.61 3 1 5 1.45 63.94 4 2 0 1.4564.24 2 3 2 1.45 64.27 4 0 4 1.44 64.55 2 1 6 1.44 64.88 4 2 1 1.4365.01 2 2 5 1.43 65.42 3 2 4 1.41 66.00 4 2 2 1.39 67.27 4 1 4 1.3967.50 0 2 6 1.38 67.81 2 3 3 1.38 68.05 3 3 0 1.37 68.63 5 1 0 1.3768.69 0 1 7 1.36 68.78 1 3 4 1.36 68.89 1 2 6 1.36 69.26 3 3 1 1.3569.37 5 1 1 1.35 69.43 1 1 7 1.34 70.22 4 2 3 1.33 70.97 4 0 5 1.3271.26 3 3 2 1.32 71.57 5 1 2 1.32 71.62 2 0 7 1.32 71.65 *peaks on topof the main peaks of the anatase.

interplanar distances for CaTi₅O₁₁; Space group: Cmcm a = 3.8 Å, b =12.1 Å, c = 17.7 Å h k l d °2θ 0 0 2 8.85 9.99 0 2 0 6.04 14.66 0 2 15.71 15.50 0 2 2 4.99 17.77 0 0 4 4.43 20.05 0 2 3 4.22 21.04 1 1 0 3.6124.62 0 2 4 3.57 24.93 1 1 1 3.54 25.13 1 1 2 3.35 26.63 1 1 3 3.0828.95 0 2 5 3.05 29.22 0 4 0 3.02 29.58 0 4 1 2.97 30.01 0 0 6 2.9530.27 0 4 2 2.86 31.29 1 1 4 2.80 31.95 1 3 0 2.76 32.44 1 3 1 2.7232.84 0 4 3 2.69 33.32 0 2 6 2.65 33.79 1 3 2 2.63 34.02 1 1 5 2.5335.47 1 3 3 2.50 35.92 0 4 4 2.49 35.99 1 3 4 2.34 38.43 0 2 7 2.3338.57 0 4 5 2.30 39.19 1 1 6 2.29 39.40 0 0 8 2.21 40.75 1 3 5 2.1841.47 0 4 6 2.11 42.83 0 2 8 2.08 43.53 1 1 7 2.07 43.65 1 5 0 2.0444.47 1 5 1 2.02 44.78 1 3 6 2.01 44.96 0 6 0 2.01 45.02 0 6 1 2.0045.33 1 5 2 1.98 45.69 0 6 2 1.96 46.24 0 4 7 1.94 46.83 1 5 3 1.9247.19 0 6 3 1.90 47.72 2 0 0 1.89 48.01 1 1 8 1.89 48.19 0 2 9 1.8748.65 1 3 7 1.86 48.82 2 0 2 1.85 49.17 1 5 4 1.85 49.23 0 6 4 1.8349.74 2 2 0 1.81 50.48 2 2 1 1.80 50.76 0 4 8 1.78 51.15 2 2 2 1.7751.59 0 0 10 1.77 51.59 1 5 5 1.76 51.76 0 6 5 1.75 52.26 2 0 4 1.7452.53 2 2 3 1.73 52.96 1 1 9 1.73 52.96 1 3 8 1.73 53.02 0 2 10 1.7053.94 1 5 6 1.68 54.74 2 2 4 1.67 54.84 0 6 6 1.66 55.22 0 4 9 1.6555.74 2 2 5 1.61 57.20 2 4 0 1.60 57.41 1 3 9 1.60 57.51 2 4 1 1.6057.66 2 0 6 1.59 57.82 1 1 10 1.59 57.97 1 5 7 1.59 58.13 2 4 2 1.5858.43 0 6 7 1.57 58.59 1 7 0 1.57 58.79 1 7 1 1.56 59.04 0 2 11 1.5559.39 2 4 3 1.55 59.70 1 7 2 1.55 59.80 2 2 6 1.54 60.00 0 4 10 1.5360.60 1 7 3 1.52 61.05 0 8 0 1.51 61.39 2 4 4 1.51 61.44 0 8 1 1.5061.64 1 5 8 1.50 61.88 1 3 10 1.49 62.28 0 6 8 1.49 62.33 0 8 2 1.4962.38 1 7 4 1.48 62.77 0 0 12 1.48 62.96 2 2 7 1.47 63.20 1 1 11 1.4763.20 0 8 3 1.46 63.60 2 4 5 1.46 63.64 2 0 8 1.44 64.75 1 7 5 1.4364.94 0 2 12 1.43 65.04 0 8 4 1.43 65.28 0 4 11 1.42 65.71 1 5 9 1.4165.99 2 4 6 1.41 66.28 0 6 9 1.41 66.42 2 2 8 1.40 66.80 1 3 11 1.3967.31 0 8 5 1.39 67.41 1 7 6 1.39 67.55 2 6 0 1.38 67.93 2 6 1 1.3768.16 1 1 12 1.37 68.67 2 6 2 1.36 68.86 2 4 7 1.35 69.32 0 8 6 1.3469.98 2 6 3 1.34 70.02 1 5 10 1.34 70.43 1 7 7 1.33 70.57 2 2 9 1.3370.75 0 6 10 1.33 70.85

Example 3 Microscope Analysis

In order to better understand the nature of the sample, both the sampleas such and the acid residue, were subjected to analysis by transmissionelectron microscopy (TEM). The observations allowed to establish thatthe sample is composed of a mixture of crystals of a few microns ofcalcium carbonate, surrounded by micro-nano crystalline aggregates(grains) of calcium titanate and carbonate having variable size from 10nm to 150 nm (see FIG. 4).

Through microanalysis with EDS detector it has been possible toidentify, as majority constituents, two families of crystals, onecontaining exclusively Ti, which proved to be anatase crystals, and onewith a slightly rounded shape containing both Ca and Ti. Thesemiquantitative analyses made by focussing the electron beam ondifferent crystals of the latter phase have allowed to establish thatthe Ca:Ti ratio in this phase, here referred to as CT2, is about 1:2

High-resolution images were carried out on some crystals of this phasewith the corresponding Fourier transforms (FIGS. 5-8), from which it hasbeen possible to extract information on the cell parameters for the CT2phase:

Orthorhombic: a=7.1 Å, b=5.0 Å, c=9.9 Å.

The extinction conditions observed are the following:

0K1 k+1=2n

hh1 no cond  (1)

2hh1 no cond

h00 h=2n

0k0 k=2n  (2)

By adding the extinctions (1) and (2) one obtains as possible spacegroup Pna21 (Herman Mauguin Symbol), corresponding to the space group 33shown in International Tables of Crystallography, vol. A, “Space GroupsSymmetry”, V ed., Kluver Acad. Publ. 2002)

Possible monoclinic distortions can exist and the TEM data obtainedcannot exclude them.

The software used for simultaneously indexing such patterns was QED(Belletti D., Calestani G., Gemmi M, Migliori A.—QED V1.0: a softwarepackage for quantitative electron diffraction datatreatment—Ultramicroscopy, 81 (2000) pp 57-65).

In light of the information obtained on the cell of this new phase itwas possible to assign some of the peaks not identified in thediffractogram of the sample STCA06 to the CT2 phase.

The remaining peaks are attributable to a different phase (CTS, seelater).

The cell parameters of the CT2 phase have been further refined throughfitting of the calculated diffractometric profile with the real one.

Through microanalyses with EDS detector it has been confirmed that thefamily of rounded crystals is conform with CT2 phase, found in thesample of photocatalytic composite.

Other crystals of characteristic elongated form (FIG. 9) were found tocontain Ca, Ti and small amounts of Na. This new crystalline phase,characterised by a Ca:Ti ratio of about 1:5, is referred to here as CTS.Similarly to what was done for CT2, some high-resolution images weretaken, with the corresponding Fourier transforms (see FIG. 10-15) fromwhich it has been possible to extract information on the cellparameters.

The main characteristic of this phase is a periodicity of 17.6 Å.

From the simultaneous indexing of such patterns via QED software(Belletti et al., op. cit.) it has been possible to derive a possiblecell for the compound in question. The cell was orthorhombic C centred:

a=3.75 (10) Å, b=11.85 (20) Å, c=17.6 (2) Å (decimal error)

Possible monoclinic distortions can exist and the TEM data obtainedcannot exclude them.

The extinction conditions observed are:

hk1 h+k=2n

hk0 h+k=2n

0k1 not able to be determined

h01 h,1=2n

h00 h=2n

0k0 k=2n

001 1=2n

These extinctions are compatible with the following possible spacegroups:

type C-c-: Cmc21, C2 cm, Cmcm (corresponding to the space group 63, cf.International Tables of Crystallography, vol. A, “Space GroupsSymmetry”, V ed., Kluver Acad. Publ. 2002) in the case of extinction 0k1k=2n;type Ccc-:Ccc2, Cccm in the case of extinction 0k1 k,1=2n.

The cell parameters of the CT5 phase have been further refined throughfitting of the calculated diffractometric profile with the real one.

Example 4 Analysis of Specific Bet Surface and Microporosity

The values measured during the analysis of the new photocatalyticcomposite STCA 06 shown in the table show an increase in the specificsurface of the heat treated product, with respect to the limestone assuch, with a substantial increase in the non-microporous fraction

S.S.A Micropore S.S.A non- BET Volume Micropores micropores m²/g mm²/gm²/g m²/g STCA 06 24.69 0 0 24.69 CA - 0.84 0.01 0.03 0.81 limestone

The analysis of the influence of the heat treatment on the specificsurface (FIG. 1), carried out on the sample of photocatalytic composite,shows a linear decrease with the temperature as shown in the tableaccording to the function:

BET = −0.4312 × T + 302.61 (m²/g) 450 < T < 650 Calcining T ° 450 550650 BET m²/g 110.93 60.72 24.69

Example 5 Photocatalytic Activity on Cement NO_(x) AbatementMeasurements

The composite STCA06 was mixed with white cement (Italbianco 52.5 diRezzato) so as to obtain photocatalytic cements with percentage byweight of photocatalyst within the range 2.5-8.5%. NO_(x) abatementanalyses were carried out on cement mortars made with normalised sandCEN (according to UNI 196-1) by preparing tests in Petri dishes ofdiameter 8 cm and surface of about 60 cm². The results obtained show anexcellent behaviour of such cements, comparable to that of cementcontaining commercial anatase (FIG. 16).

The abatement values measured on the mortars CEN containing thecomposite STCA06 at different concentrations on cement have shown goodNO_(x) abatement values already at percentages of around 2.5% by weight.(See FIG. 17)

The treatment temperature proved important for photocatalytic activity.Indeed, it was observed that there is a progressive increase in activityas the temperature increases, as highlighted by observations at 450, 550and 650° C. (FIG. 18).

Example 6 Photocatalytic Activity on Cement VOC Abatement Measurements

The evaluation of the abatement capability of aromatic hydrocarbons wascarried out using the pure photocatalytic products (not mixed withcement) under UV radiation. Ethylbenzene was used as organic substance,using a flow apparatus typical of tests on catalysts (oxidation ofethylbenzene in air). In this way the intrinsic activity of the materialis determined, disregarding the diffusive phenomena. The resultsobtained show an excellent level of abatement activity of the product.It is even greater than the best commercial TiO₂ (FIG. 19).

1. Photocatalytic composite comprising limestone, titanium dioxide, andcalcium titanate in the crystalline phases CT2 and/or CT5 characterisedby the following diffraction peaks: CT2: (002) d=4,959; (210-202)d=2,890; (013) d=2,762 and (310-122) d=2,138; CT5: (002) d=8,845; (023)d=4,217; (110) d=3,611 and (006) d=2,948.
 2. Composite according toclaim 1, wherein said peaks of the CT2 phase are indexed with anorthorhombic cell having the following reticular parameters: a=7.1 Å,b=5.0 Å, c=9.9 Å.
 3. Composite according to claim 1, wherein said peaksof the CT5 phase are indexed with an orthorhombic cell having thefollowing reticular parameters: a=3.8 Å, b=12.1 Å, c=17.7 Å. 4.Composite according to claims 1-3, wherein the calcium titanate in CT2phase has empirical formula CaTi₂O₅, and the calcium titanate in CT5phase has empirical formula CaTi₅O₁₁.
 5. Composite according to claims1-4, wherein the CT2 phase is present in amount higher than the CT5phase.
 6. Composite according to claims 1-5, having specific BET surfaceranging from 10 to 150 m²/g.
 7. Composite according to claim 6, havingspecific BET surface ranging from 15 to 50 m²/g.
 8. Composite accordingto claim 7, having specific BET surface ranging from 20 to 30 m²/g. 9.Calcium titanate with high photocatalytic activity, characterised by thepresence of the crystalline phases CT2 and/or CT5, as described inclaims 1-5.
 10. Process to obtain the composite described in claims 1-8,comprising limestone and a precursor of titanium dioxide in presence ofa basic aqueous solution, recovering the solid product thus obtained,washing it until neutrality, drying it and calcining it.
 11. Processaccording to claim 10, wherein the precursor is titanyl sulphate, thebasic solution contains NaOH, and the solid product is calcined at atemperature ranging from 300 to 800° C.
 12. Process according to claim11, wherein the solid product is calcined at a temperature ranging from450 to 700° C.
 13. Photocatalytic composite obtainable by the processdescribed in claims 10-12.
 14. Use of a composite as described in claims1-8, in preparing an article of manufacture having photocatalyticactivity.
 15. Use according to claim 14, wherein the article ofmanufacture contains the composite dispersed in bulk.
 16. Use accordingto claim 14, wherein the article of manufacture contains the compositelayered on at least part of its external surface, as a coating element.17. Cement composition comprising the photocatalytic composite describedin claims 1-8, water, a hydraulic binder, and optionally aggregates. 18.Dry premix comprising the photocatalytic composite described in claims1-8, a hydraulic binder, and optionally aggregates.
 19. Photocatalyticarticle of manufacture comprising, dispersed in bulk or layered on itssurface, the composite described in claims 1-8.